Test objects larger than the field of view of optical measuring instruments, such as interferometers, can be measured by combining individual optical measurements, referred to as “sub-aperture measurements”, for constructing a composite optical measurement of the test object, presumably a “full-aperture measurement”. The process of combining the individual optical measurements is referred to as “stitching”. Data is typically collected from the individual measurements in the form of local data maps, which are “stitched” together to form a composite global data map. Two main approaches to “stitching” are used, both based on exploiting redundant data within regions of overlap between adjacent measurements.
One of the two main approaches to “stitching” starts with a first local data map and determines differences from an adjacent second local data map within a region of overlap between the two local data maps. The orientation of the second local data map is adjusted with respect to the first local data map to minimize differences within the overlap region. Any remaining differences within the overlap region are averaged to complete a junction between the two local data maps and create a combined map. The local data maps collected by additional measurements are similarly joined to the combined data map, one at a time, to produce a composite global data map. The sequential assembly of the composite global data map leads to cumulative errors, which propagate through assembly generations, and the results are dependent on the order at which the local data maps are stitched together.
The other of the two main approaches provides for collectively minimizing differences within all of the regions of overlap between the local data maps by iterating over ranges of possible individual local data map orientations. The progress of each iteration is checked against the resulting changes to the differences within the overlap regions. The iterative solution specifies orientations of each of the local data maps for assembling a composite global data map. Although good results are possible, processing time, particularly for large numbers of measurements, can exceed reasonable wait times for measuring parts in succession. Some iterative stitching solutions can require one hour or even several hours of processing.